1. Field of the Invention
The present invention relates generally to reference signal producing circuits, and more particularly to improvements in the reference signal producing circuit for producing a reference signal for phase servo in synchronization with an external signal such as a composite synchronizing signal in a motor servo circuit of a video apparatus such as a video tape recorder (hereinafter referred to as a VTR).
2. Description of the Background Art
Conventionally, there is provided a motor servo circuit for controlling rotation of a cylinder motor and a capstan motor in a VTR. In such a motor servo circuit, a reference signal for controlling phase servo of the motor is produced based on a composite synchronizing signal extracted from a video signal. This production of the reference signal is in general carried out by employing a synchronization separator circuit for separating a vertical synchronizing signal from the composite synchronizing signal. As an example of this synchronization separator circuit, a circuit is proposed and also disclosed in Japanese Patent Laying-Open No. 61-288574, which includes an up/down counter and thus controls counting operation of this counter in accordance with the level of the composite synchronizing signal to sense the vertical synchronizing signal depending on an output state of this counter.
Furthermore, this synchronization separator circuit carries out separation of a horizontal synchronizing signal for special reproduction in the VTR, which will be described later.
As another example of the conventional synchronization separator circuit, a circuit is proposed which has a compensating function in the case of incidental one-shot lack of pulses of the vertical synchronizing signal and the horizontal synchronizing signal within the composite synchronizing signal.
However, no circuit has the compensating function in the case of successive lack of the plurality of horizontal synchronizing signal pulses or vertical synchronizing signal pulses within the composite synchronizing signal. Therefore, various problems arise as follows in the case of the successive lack.
In the case of changing channels during picture recording in the VTR, for example, supply of the video signal is generally interrupted to lose the vertical synchronizing signal and thus synchronization reference for motor servo. Furthermore, in the case of only recording a sound in a high fidelity video or the like, or in the case of being unable to sense the vertical synchronizing signal because of an extremely weak broadcasting signal, the vertical synchronizing signal pulses lack successively, thereby not obtaining the synchronization reference for phase servo of various types of motors.
In addition, the following problem occurs in the special recording in the VTR, in the case that the horizontal synchronizing signal pulses in the composite synchronizing signal successively lack from the above described causes. That is, in the special reproduction in the VTR, V jitter (very fine movement of vertical sync) on a screen is prevented by employing a quasi vertical synchronizing signal in which the plurality of horizontal synchronizing signal pulses are inserted immediately before the vertical synchronizing signal pulse; however, the V jitter cannot be prevented when the horizontal synchronizing signal pulses in the composite synchronizing signal lack successively.